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Structure-function studies on Acanthamoeba myosins IA, IB, and II
E D Korn1, M A Atkinson, H Brzeska
1Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, Bethesda, Maryland 20892.
Abstract:
Myosins IA and IB are globular proteins with only a single, short (for myosins) heavy chain (140,000 and 125,000 daltons for IA and IB, respectively) and are unable to form bipolar filaments. The amino acid sequence of IB heavy chain shows 55% similarity to muscle myosins in the N-terminal 670 residues, which contain the active sites, and a unique 500-residue C-terminus highly enriched in proline, glycine, and alanine. The C-terminal region contains a second actin-binding site which allows myosins IA and IB to cross-link actin filaments and support contractile activity. Myosins IA and IB are regulated solely by phosphorylation of one serine on the heavy chain positioned between the catalytic site and the actin-binding site that activates ATPase. Myosin II is a more conventional myosin in composition (two heavy chains and two pairs of light chains), heavy chain sequence (globular head 45% identical to muscle myosins and a coiled-coil helical tail), and structure (bipolar filaments). The tail of myosin II is much shorter than that of other conventional myosins, and it contains a 25 amino acid sequence in which helical structure is predicted to be weak or absent. The position of this sequence corresponds to the position of a bend in the monomer. Myosin II heavy chains also have a 29-residue nonhelical tailpiece which contains three regulatory, phosphorylatable serines. Phosphorylation at the tip of the tail regulates ATPase activity in the globular head apparently through an effect on filament structure.
Insights
Myosins IA and IB possess unique C-termini enabling actin cross-linking and contraction, regulated by heavy chain phosphorylation. Myosin II, a conventional myosin, also shows unique tail features impacting ATPase activity via phosphorylation.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Myosins are motor proteins crucial for cellular processes.
- Myosins IA and IB differ significantly from conventional myosins in structure and function.
- Myosin II represents a more conventional myosin type with distinct structural characteristics.
Purpose of the Study:
- To elucidate the structural and functional differences between myosins IA, IB, and II.
- To investigate the role of unique C-terminal domains in myosins IA and IB.
- To understand the regulatory mechanisms, particularly phosphorylation, in these myosin types.
Main Methods:
- Amino acid sequence analysis comparing myosins IA, IB, and muscle myosins.
- Identification and characterization of actin-binding sites.
- Analysis of phosphorylation sites and their regulatory effects on ATPase activity.
Main Results:
- Myosins IA and IB have unique C-terminal domains with a second actin-binding site, enabling filament cross-linking and contraction.
- These myosins are regulated by phosphorylation of a single serine on the heavy chain, activating ATPase.
- Myosin II exhibits a conventional structure but with a shorter tail containing a potential bend and regulatory phosphorylation sites.
Conclusions:
- Myosins IA and IB possess distinct structural adaptations for actin interaction and regulation.
- Phosphorylation plays a key role in modulating the activity of both conventional and unconventional myosins.
- Understanding these myosin variations provides insight into diverse cellular motor functions.